The discovery that compost may have therapeutic powers has astonished scientists

Anyone with a garden knows how to make compost. But who can turn "green" waste material into a product with an extraordinary natural power to suppress plant diseases such as club-root in brassicas, brown rot in potatoes and red core in strawberries?

The answer is Eco-Sci, a small company based in Exeter. Although barely three years old, the firm has established itself as a pioneer of composting techniques in the United Kingdom, and has stumbled on a fact which may prove of global significance.

The discovery that compost has therapeutic powers astonished the firm's scientists. It emerged from field trials in which crops treated with compost not only grew faster but also seemed more resistant to disease. Maize, for instance, produced leaves double the normal size, bigger cobs, and stems less prone to topple over.

Nobody yet understands quite why the compost is so effective. Laboratory research is in progress both at Eco-Sci and at Exeter University, with back-up work going on at the government's Central Science Laboratory; but tests have yet to establish whether the compost contains chemicals or organisms which are actually killing harmful bacteria, or whether, by promoting rapid root growth and thicker cell walls, it is merely furnishing plants with better resistance to disease.

Eco-Sci's raw material - 20,000 tons a year - comes from gardens and municipal parks in Devon, and the handling of it is an impressive operation. A large mobile shredder tours four collection sites and reduces great heaps of branches, shoots, leaves and grass to a coarse pulp. As each load goes into the spinning flails of its jaws, the machine gives an angry rumble, loud as thunder, and spews out pieces of wood to a distance of 60 or 70 yards.

The pulp is then laid out in tapered windrows or banks, four metres wide at the base and 50 metres long. There it remains for about three months, cooking gently, having its temperature taken once a week and being turned by a special machine every fortnight or so, depending on how wet or dry the weather has been.

Finally it is put through a screen, which separates out undigested lumps, and the good stuff goes on to a big pile, under cover, to mature. By then it looks and smells like fine, dry earth.

The company has begun marketing West Country Compost at a retail price of about pounds 2.45 for a 50-litre bag. Yet Eco-Sci's main income is derived from receiving raw material in the first place.

Problems of waste-disposal are already acute, and Devon local authorities pay a substantial gate-fee for every ton of waste dumped at Eco-Sci's sites. In the view of Tom Young, the company's managing director, "there is soon going to be a desperate shortage of holes in the ground", and he believes that the cost of dumping rubbish, now pounds 20 a ton in many areas, will double over the next few years.

Professional compost-making is thus pressured by an urgent need for innovation, and Eco-Sci is experimenting with new methods of processing ordinary household waste in giant plastic bags known as Eco-pods. Domestic rubbish arrives at its depot in Plymouth by the truck-load. In theory, the garbage has already been sorted by householders into organic and non-organic categories, but the heap I saw being handled, under a swarm of gulls, contained many choice allegedly-putrescible items such as loudspeakers, vacuum cleaners and pairs of trainers.

With these removed by hand, the bulk goes into a shredder-scruncher, and thence is carried by conveyor belt to a rotary trommel screen, which separates out any surviving pieces 25mm or more across. The fine material, known as feedstock, is loaded by a self-propelled stuffer into a pod - a tube of heavy-duty green plastic 10ft in diameter.

As the stuffer creeps forward, the pod gradually extends and fills like a giant sausage. At its maximum length of 60 metres, one unit can hold 200 tons, and it has numerous advantages over open-air systems. Not only does it contain smells, dust and potential leakage; because temperature is accurately controlled by air blown through the pod, the process of decomposition is accelerated, finishing within eight to 12 weeks. The compost which emerges cannot be sold to gardeners, because it may contain chips of glass; but it is perfectly adequate for landscaping over filled- in sites.

So the race is on to recycle everything that can be saved, to dump as little as possible in the ground. Yet it is compost's natural ability to fight plant disease which most excites Eco-Sci's staff.

Already they have conducted field trials in Hungary and India, and on Thursday two senior executives returned from an exploratory visit to Egypt. There, it is hoped, their breakthrough may prove a decisive factor in the battle against brown rot, which has become endemic in the country's vital potato crop.